let code

function createCode() {
    code = "";
    let codeLength = 4;//验证码的长度
    let checkCode = document.getElementById("code");
    let random = [0,1,2,3,4,5,6,7,8,9,'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R',
        'S','T','U','V','W','X','Y','Z'];//随机数
    for(let i = 0; i < codeLength; i++) {//循环操作
        let index = Math.floor(Math.random()*36);//取得随机数的索引（0~35）
        code += random[index];
    }
    checkCode.value = code;//把code值赋给验证码
}